The field of the present invention relates to etching of silicon nitride and/or silicon oxide by anhydrous HF.
1. State of the Art
Existing solutions for etching comprise aqueous solution etching, plasma etching and others such as nonaqueous solution etching and plasmaless gas phase etching.
Methods for etching Si3N4 are described in several documents. U.S. Pat. No. 5,534,107 describes a method for removing silicon nitride from a substrate. The substrate (being silicon or silicon oxide) is placed in a gaseous environment comprising a fluorine comprising gas which can be photodissociated using UV-radiation. EP 0590876 describes a selective silicon nitride etch (with respect to silicon oxide) using an etch bath comprising phosphoric acid, nitric acid and hydrofluoric acid. EP 0706206 describes a method for selectively etching Si3N4 to SiO2. A plasma etch system comprising CF4, O2 and Ar is used.
The document U.S. Pat. No. 5,420,078 describes a method for forming via holes in a dielectric layer such as an Si3N4 or SiO2 layer using an acidic vapour at room temperature and atmospheric pressure.
2. Aims of the Invention
The present invention aims at providing a new method for etching silicon nitride and/or silicon oxide effectively from a silicon wafer substrate. A further aim of the present invention is to provide a means for selectively etching silicon nitride in the presence of silicon oxide layers.
The invention comprises a method for removing silicon nitride from a substrate by etching, characterised in that it comprises the following steps:
bringing the surface temperature of said substrate to at least 125xc2x0 C., and
contacting said substrate with anhydrous hydrogen halognide,
and characterised in that the silicon nitride etching rate is at least 20 nm/min.
Said method can be further characterised in that the pressure while etching lies between 10 and 760 torr. Preferably, the substrate surface temperature while etching lies between 125 and 300xc2x0 C. Further, said hydrogen halogenide is preferably chosen from the group consisting of HF and HCl.
The method of the invention can further comprise the following steps:
loading a substrate in a process chamber;
heating up said substrate to process temperature while pumping the chamber down to vacuum;
etching of silicon nitride at constant process temperature, by filling said process chamber with HF until the target pressure is reached;
vaporization of the reaction product;
cooling down the wafer
filling the chamber to atmospheric pressure with N2; and
unloading said substrate.
In a preferred embodiment, the substrate is a silicon wafer.
The method can further be characterised in that it comprises the formation of a halogen salt, The method is is characterised in that the molten halogen salt is an ammonium salt.
Said molten halogen salt is preferably an ammonium salt. Further, said halogen is preferably chosen from the group consisting of F and Cl.
In an advantageous embodiment, said salt is selected from the group consisting of (NH4)2SiF6, NH4HF2, NH4HF and NH4Cl.
Preferably, said salt is formed by reaction of HF and/or HCl with Si3N4.